It is not so much a question of modern or legacy but of intended audience. The OS(or more specifically the desktop environment) intended for a general audience

It is not so much a question of modern or legacy but of intended audience. The OS(or more specifically the desktop environment) intended for a general audience will be more bland, uniform and legacy than the one made for a smaller more specific audience, perhaps it was made for the sole use of the author and they decided to share.. The general audience will have a preconceived expectation, you might say an intuition, about how the environment works, this mean, that an environment intended for such an audience cannot really have too much advancement or it will drift too far from expectation.